manually editing postfix_header_checks

hi,

when i edit /opt/zimbra/conf/postfix_header_checks to remove some header informations it works but only for the next reload or restart of the server!
where did i have to edit postfix_header_checks that the changes are still there when i restart or reload the server?
sample line to add to postfix_header_checks: /^X-Mailer:/ IGNORE